1. Field of the Invention
This invention generally relates to a control system located at a first site for guiding a steerable object from that site toward a second site. More specifically this invention relates to such a control system that is operable even when both the first and second sites undergo independent motion.
2. Description of the Prior Art
Submarines include a control system for guiding an acoustic homing torpedo launched from the submarine toward a target. In this particular application, the torpedo constitutes a steerable object while the submarine and target constitute first and second sites, respectively, that are capable of undergoing independent motion. The control system used in this application is oftentimes a "beam rider" control system at the submarine, or first site, that guides a torpedo toward the target, as the second site, along a bearing line to the second site, particularly when the range to the target is not known.
Beam rider control systems generally operate with steerable objects characterized by some internal homing or equivalent steering control system. In the case of a submarine launched torpedo, the beam rider control system directs a torpedo with an acoustic homing system toward the target. When the torpedo comes within the effective range of the homing system, the homing system takes over the steering control function. Steerable objects with such homing systems are also characterized by having an external point in front of the steerable object called a "guidance point". This guidance point corresponds to the centroid of the acoustic beam in the case of a torpedo with an acoustic homing device. Generally a beam rider control system operates to maintain the guidance point of a steerable object, such as a torpedo, on a bearing line from the first site to the second site.
Prior art beam riding control systems for submarine launched torpedoes include a control mechanism, a torpedo model and a communications link to the torpedo, as the steerable object. The torpedo model is a mathematical replica of the torpedo that provides position and status information for post launch guidance operation. The control mechanism utilizes measured contact information, particularly a bearing from the submarine to the target, and torpedo model information, particularly the bearing from the submarine to the guidance point of the torpedo, to generate a command sequence for maintaining the guidance point on a trajectory to the target. These commands are transferred from the submarine to the torpedo through a wire communications link.
Present beam rider control systems do not include any mechanism for readily allowing the inclusion of heuristic information into the control system, particularly information about expertise gained through past experience with manual beam rider guidance systems. Also present beam rider control systems normally require an operator to determine whether to issue a particular command to a torpedo. Present systems do not automatically generate and issue guidance commands in a continuous fashion.
Other approaches for directing a steerable object from a launch site to a target involve complicated control systems based on sets of differential equations and estimates of input parameters. Such systems operate in response to analytical controllers. However, like prior art beam rider control systems such analytical controllers are not readily adapted to utilize expert knowledge gained through experience.
U.S. Pat. No. 5,101,351 depicts an alternative control system, known as a "fuzzy control" system for use in steering a vehicle. In this system a camera forms an image of the road. An image processor uses the image to calculate the deviation between a plurality of reference points on a road and the direction in which the vehicle is traveling. Each reference point is associated with a plurality of visual points spaced along the road at predetermined distances ahead of the vehicle. The control determines the product of the deviation for each visual point and a membership function indicating the degree of importance attached to each visual point. The membership functions are varied in accordance with the time rate of change of the deviations. The control also calculates a total deviation equal to the sum of the products, and this total deviation is the basis for steering control.
Several features of the control system in U.S. Pat. No. 5,101,351 are antithetical in applications such as submarine launched torpedo guidance applications. The entire control system disclosed in the patent is located on a vehicle used as a steerable object; in a submarine application, the control system must be located on the submarine remotely from the torpedo as a steerable object. Moreover in a submarine application, both the submarine and target usually undergo motion relative to each other and relative to the torpedo. The system disclosed in the patent monitors only a fixed line on a road so it only needs to respond to deviations in the direction of vehicle travel relative to the reference line. Finally, the fuzzy control system described in the patent selects control rules on the basis of one set of related parameters, namely deviation and a derived rate of change of deviation that is modified by vehicle speed. Such a system uses a single set of control rules derived from a single input; in a submarine application the system control rules are derived from two inputs. The first input pertains to the vehicle guidance point. The second input pertains to the vehicle. Consequently a system based upon U.S. Pat. No. 5,101,351 will not work well in a submarine environment.